After the United States entered the war, his friend and coworker, Frederick Kilgour, went to Washington to work for the OSS, which actively recruited scholars and specialists from the Ivy League. Kilgour’s assignment was to organize an overseas program to acquire enemy publications. He needed seasoned librarians, he wrote his former boss, Harvard Library director Keyes Metcalf, and preferred men ineligible for the draft, “a mild 4-F such as Reuben,” whose academic knowledge, library experience, and fluency in several languages made him an obvious choice. Metcalf endorsed the suggestion: “I know that he is restless and anxious to do his share in the war.”
Reuben Peiss jumped at the chance to join the OSS and travel abroad. Arriving in Lisbon in September 1943, he quickly picked up the language and became attuned to the “machine gun” rate of speaking. The city was remarkable in its difference from every place he had known—its narrow streets and steep hills, the day full of sunlight and “night fragrant, with star-studded skies.” He marveled at the courtesy of the Portuguese, the strong family feeling, even the women with “enormous baskets of fish on their heads, striding along with magnificent posture.” He also saw the poverty and “deplorable squalor” behind the picturesque scene, and the “very deep strain of sadness running throughout life here.” In Lisbon he acquired publications from Germany and occupied countries, an open activity that at times drifted into clandestine work. At the end of 1944, when the border between Switzerland and France opened up, he went on to the OSS post in Bern, with trips to London, Paris, and Geneva. “I have been seeing the world, and it has been an exciting experience indeed,” he wrote his aunt.
Few letters record how he felt about those experiences or reveal his inner life. He remained single over the years; family members fleetingly mention a college sweetheart and a rumored affair with a Parisian woman, but nothing more. His feelings occasionally overcame him—his outrage over perceived antisemitism in the treatment of his brother, and the pain, visible in his handwriting, when his young sister died of leukemia. But his correspondence usually moved along easier registers—work, books, people, and immediate problems to solve.